The invention relates to frequency hopping (FH) radio systems. In particular, it relates to FH radio systems applied in a multi-cell or cellular application comprising fixed base stations and moving portables that while moving, connect from one base station to the other. The invention enables handovers to take place between FH base stations even if the base stations are by no means coordinated.
Since the widespread use of mobile telephony, cellular systems are well known and have reached a high level of maturity. Cellular systems typically comprise a mobile network with a number of base stations located at strategic positions, each base station covering a restricted area called a cell. Because adjacent cells partly overlap, a portable device can move from one cell to the other without losing contact with the mobile network. As the portable moves during a call, the connection is handed off from one base station to another, depending on the relative location of the portable with respect to the base stations.
To support the access to the network and the handover function, the base station typically transmits a predefined (known) radio signal, the so-called control channel or beacon. The control channel reveals the presence of the base station to the portable. Based on the signal strength of the control channel received in the portable, a decision can be made regarding which base station the portable should connect to before or during a call.
Control channels are fixed channels that can easily be found by the portables. They either use a dedicated frequency and/or time slot in Frequency Division Multiple Access (FDMA)- /Time Division Multiple Access (TDMA)-based systems, or a fixed spreading code in direct-sequence Code Division Multiple Access (CDMA)-based systems. In all cases, the carrier frequency on which the control channel is located is fixed, although it can differ per base station. All the portable has to do is to tune to the proper carrier frequency and scan the signals until a decodeable signal is found.
In systems based on frequency-hop CDMA, the situation is completely different. In these systems, the carrier frequency changes periodically according to a pseudo-random hop sequence. Therefore, it is difficult to build cellular systems with a FH system because FH control channels are difficult for the portable to find, especially when the portable has no knowledge of the hop timing and the hop sequence. In the past, FH has been applied in digital cellular systems like GSM in order to increase capacity through interference diversity; however, in these systems, it is only the traffic channels that hop, and never the control channels.
The search effort of a portable in a FH cellular system can be reduced considerably if the hopping between the base stations is coordinated. In this case, once the portable has synchronized to one base station, little effort is required to lock to other, nearby base stations, provided that their FH transmissions are coordinated with the current base station. Thus, the search effort in coordinated systems is only expended the first time the portable enters the mobile network, such as at power up when turning on the portable.
However, when the radio interface uses certain frequency bands, regulations and rules set by regulatory bodies, like the Federal Communications Commission (FCC) in the U.S. or the European Telecommunications Standard Institute (ETSI) in Europe, prohibit the coordination of base stations. An example is the unlicenced Industrial/Medical/Scientific (ISM) band at 2.4 GHz. To use this band, the radio system must spread its signals either by direct-sequence (DS) spreading, or by frequency-hop spreading. As was described above, cellular systems based on DS spreading are attractive to use because fixed carrier frequencies can be used to support the control channels. However, FH spreading is more robust in environments with unknown jammers and results in cheaper transceiver implementations. Unfortunately, the set of rules governing the ISM band do not permit the coordination of FH base stations. This hinders the deployment in this band of multi-cell and cellular FH systems that support, for example, handovers.
Even if a radio band is used in which regulations allow coordination of base stations, for low cost solutions coordination is undesirable. Base stations being used as access pints for wireless access to an existing wired network like PSTN, Ethernet, or any other conventional LAN, cannot rely on coordination and synchronization signals on the wireline. In such a case, a dedicated network would be required to deal with the mobility functions required by the access points. This is not an attractive solution. In the preferred case, an access point can be plugged in to a conventional wired backbone without requirements for coordination and synchronization signals.
It is therefore desirable to provide a system and method that applies frequency hopping in a communications system that performs handovers between uncoordinated base stations.
It is therefore an object of the invention to provide an uncoordinated frequency hopping cellular system, a mobile unit for use in an uncoordinated frequency hopping cellular system, a base station for use in an uncoordinated frequency hopping cellular system, and methods for operating the mobile unit, the base station and the system.
In accordance with one aspect of the invention, the mobile unit determines information pertaining to a set of base stations that are within a geographical region defined by a location of the mobile unit. The mobile unit then supplies, to at least one of the base stations in the set, the information pertaining to at least one other base station in the set. In each base station, this information may be received and accumulated.
In another aspect of the invention, the information may include address information corresponding to the set of base stations.
In yet another aspect of the invention, the information may include clock offset information representing a difference between a clock value associated with said at least one of the base stations and a clock value associated with at least one of the other base stations in the set.
In still another aspect of the invention, mobile unit receives additional information from said at least one of the base stations, wherein the additional information includes information pertaining to at least one base station that is not in the set of base stations. For example, the addition information may include synchronization information pertaining to the at least one base station that is not in the set of base stations. This additional information may then be used to establish communication with said at least one base station that is not in the set of base stations.
The base stations may occasionally received updated information from more recent contact with mobile units. Thus, in yet another aspect of the invention, the base stations may estimate a rate of clock drift between an own clock located within the base station and another clock located within another base station in the system.
In still another aspect of the invention, the base stations may use the estimated rate of clock drift to correct a stored clock offset value representing a difference between a clock value associated with the own clock and a clock value associated with said another clock located within said another base station in the system.
In yet another aspect of the invention, a mobile unit for use in an uncoordinated frequency hopping cellular system communicates with a first base station in the uncoordinated frequency hopping cellular system; and receives information from the first base station, wherein the information includes addresses and clock offset values of neighboring base stations in the uncoordinated frequency hopping cellular system.
In still another aspect of the invention, each of the clock offset values represents a difference between a clock value associated with the first base station and a clock value associated with a corresponding one of the neighboring base stations.
In yet another aspect of the invention, the mobile unit uses the information from the first base station to establish communication with at least one of the neighboring base stations.
In still another aspect of the invention, the mobile unit supplies additional information to the first base station, wherein the additional information includes address and clock offset information pertaining to another base station in the uncoordinated frequency hopping cellular system.
In yet another aspect of the invention, the mobile unit performs an inquiry procedure that yields the additional information.